Create some new options for `BFSIterator`

[Tortar]

Fixes https://github.com/JuliaGraphs/Graphs.jl/issues/288.

This PR adds 2 new options, which can be useful when iterating in BFS:

• depth_limit: specify the maximum level at which BFS should stop beforehand.
• neighbors_type: specify which function to use when iterating in BFS, e.g. sometimes using inneighbors could be useful and this allows that

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 97.31%. Comparing base (ec6ab1b) to head (9e814be). Report is 5 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #389   +/-   ##
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  Coverage   97.31%   97.31%           
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  Files         120      120           
  Lines        6953     6956    +3     
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+ Hits         6766     6769    +3     
  Misses        187      187           

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[Tortar]

Bump, I have some use cases for this, it should be easy to review

[Tortar]

Can someone take a look?

[Tortar]

let me know if you have more review suggestions @gdalle and thank you for taking a look!

[Tortar]

Let me know if there is still something to address @gdalle

[Tortar]

If you could release a new version with this I would be grateful so that I can use it in another package